1. Field of the Invention
This invention relates to continuous navigation systems for determining the location of bodies relative to a coordinate systems, such as the WGS-72, or to other bodies. More particularly, this invention relates to low cost differential navigation systems for determining the location of potentially unstable and remote mobile units.
2. Description of the Prior Art
Both public and private navigation information services provide navigation information to the general public. For instance, LORAN-C, developed by the Department of Defense, is a government provided radio navigation service predominantly for civil marine use in the United States coastal areas. VOR, VOR/DME, and TACAN provide basic guidance for inland air navigation in the United States. Due to the large network of ground installations the coverage and availability in the United States is quite high. DECCA is a major world-wide navigation information service for air and marine users. TRANSIT is a world-wide space based navigation information service consisting of four or more satellites in approximately 600 nautical mile polar orbits. There are a number of other radiolocation services privately developed and operated, each having their area of application, strengths and weaknesses.
NAVSTAR Global Position System (GPS) is a world-wide satellite navigation information service being developed by the Department of Defense. NAVSTAR GPS should become operational between 1987 and 1989, providing position and velocity information in three dimensions, as well as precise time, to users around the globe, 24 hours a day.
NAVSTAR utilizes two frequencies. One frequency contains both "coarse" and "precise" information for navigation and positioning. The second frequency contains only the "precise" information. The "precise" information is to be available only to the military. The current plan is to make the "coarse" information available to civilian users at an accuracy level of about 100 meters.
The accuracy and precision of navigation based on any navigation service information is enhanced if the user operates in the differential mode. Generally speaking operation in the differential mode involves combining navigation information received at two different receivers where the location of one of the receivers is well-known. By combining the data, the location of the other receiver can be determined with greater accuracy than would be possible through using the data received by that other receiver alone. Using data from both receivers permits compensation for errors in the navigation system information.
Specifically, the differential mode can take many forms. One common mode of differential operation is where a reference receiver of known location takes note of the difference between its known location and its location predicted by using the navigation service information. This difference reflects errors in the information received. The errors could be deliberate errors introduced into the data for security reasons, atmospheric errors caused by the ionoshpere and troposphere, errors in the knowledge of the actual location of a navigation information service component, equipment or clock errors. These errors are key limitations on the level of accuracy and precision achievable with the navigation service. The detection and transmission to a receiver whose location is being determined permits an improvement in the level of accuracy and precision achievable.
Navigation information errors detected by a reference receiver will be largely reflected in the navigation information received by all users near the reference station. Communication of some measure of these errors to users in the vicinity of the reference receiver enhances the accuracy with which the users can calculate their location. As the distance between the user and the reference station increases, the level of enchanced accuracy achieved in the differential mode declines. This decrease is primarily due to variations in the atmosphere. Differential corrections from a reference receiver are generally useable up to a radius of 500 miles from the reference receiver. (Thus, to serve a wide geographic area, reference receivers would need to be stationed approximately every 1,000 miles.)
When the "coarse" NAVSTAR information is utilized in the differential mode it is believed that the level of positioning accuracy will improve from the 100 meter range to the one to five meter range. There are many commercial uses for a low cost, reliable navigation system with an accuracy in the one to five meter range. Many of these uses are by unstable mobile units in remote geographic locations.
Use of the differential mode requires a means for communicating differential data from a reference receiver to a user. For maximum effectiveness users should receive differential data every few seconds. For broad application of a system which includes reference receivers stationed approximately every 1,000 miles, the communication link must cover a wide geographic area, including remote regions. For commercial feasibility, the communication link must be low cost and highly reliable over long periods of continued use.
Standard radio transmissions have constituted the communications link for differential navigation systems utilized to date. However, as shown below, radio transmissions lack the low cost, the range and the high reliability over long periods of continued use to serve as a communication link for a reliable, continuous navigation system serving a wide geographic area including remote regions.
A differential navigation system communication link falls into the Federal Communications Commission classification of a radiolocation service. There are a number of frequencies that can be used for a radiolocation service. They are:
1.85--2.0 MHz PA1 216--225 MHz PA1 420--450 MHz PA1 890--942 MHz PA1 1215--1300 MHz
The frequencies at 216 MHz and higher are good for only "line-of-sight" communication. They do not meet the criteria for large area coverage. The band at 1.85--2.0 MHz would be expected to give adequate area coverage over water but not over land where the surface conductivity is low. Sky wave interference at night would affect the reliability of this band. Atmospheric conditions such as thunderstorms would also affect reliability.
The sub carrier portion of commercial FM stations or the sub audio portion of commercial AM stations could be used for the communication link. In neither case would the expected area of coverage be greater than 100 miles. Additionally, the requirements for a 24-hour/day communication link would not always be possible.
This invention solves the problem of a virtually continuous, high range, low cost, highly reliably communication link between reference units of known location and potentially unstable mobile units spread over a wide geographic area, including remote regions. This invention solves the communication link problem through specification of a commercial geosynchoronous linearly polarized earth satellite transponder as a relay for the differential navigation data to a low cost non-directional, non-stable, circularly polarized antenna located on the mobile unit. Reliable reception at the low cost, non-stable antenna is made possible through selected formatting of the data utilizing spread spectrum techniques.
The specification of an earth satellite relay satisfies the broad base requirements of the system for communication to users over a broad geographic area, including remote regions. Moreover, satellite communication links, due to their relative immunity from atmospheric disturbances, are more reliable than standard terrestrial ones.
When considered from the point of view of a network of reference receivers, a satellite communication link is also cost effective. Each reference receiver in the network can be linked to a central master earth station. The data from the various reference receivers can be multiplexed at the master earth station and then uplinked to the satellite. The satellite retransmits the data in a footprint covering, say, North America. The users of the differential correction communication link have an antenna and receiver to receive the correction terms. The user's receiver selects the data from the appropriate reference receiver and applies it to its own solution. Any number of users can use the same data from the satellite communication link.
Conventional commercial satellite receivers employ linearly polarized directional dish antennas to reliably acquire relayed data from commercial geosynchronous earth satellite transponders. According to the conventional scheme, if the user of a satellite communication link is mobile, that is subject to change in latitude, longitude or altitude, and non-stationary, that is subject to change in roll, pitch and yaw, some method must be used to keep the directional antenna pointing at the satellite. Ships that currently receive satellite data utilize expensive stabilized platforms for maintaining the proper orientation of the directional antenna. Stabilized platforms would violate this system's requirement of low cost.
According to this invention, to avoid the cost of a stabilized platform, or in situations where a stabilized platform would be impossible, the mobile user receives the relayed linearly polarized signals from the commercial satellite transponder with a low cost circularly polarized, non-stabilized, non-directional antenna. Such an antenna has a low signal to noise ratio. One would anticipate any attempt to receive commercial geosynchronous satellite transmissions with this antenna would exhibit a high data error rate. The high data error rate is virtually eliminated and the requirement of reliable reception is met, by condensing the differential data into a minimal set of information and formatting it onto the relayed signal using spread spectrum techniques.
Formatting a signal with spread spectrum techniques can yield a process gain that greatly enhances reception. However, such formatting also sacrifices data rate. Formatting for reliable reception by a circular polarized, non-stabilized, non-directional antenna results in a great sacrifice of data rate, especially when the data is relayed by a linearly polarized commercial geosynchronous earth satellite transponder.
As stated previously, high accuracy navigation requires a differential data update virtually continuously, or every few seconds. Through appropriate balancing of parameters this invention satisfies such data rate requirements. By formatting the differential data into a minimal set of information and utilizing spread spectrum techniques, this system achieves the required rate of one set of differential data relating to each of several navigation source units every few seconds. In so doing, this system maintains reliable reception of the data by low cost antennas located on mobile users spread over a broad geographic area, including remote regions.
Therefore, it is an object of this invention to provide a commercially feasible, low cost, differential navigation system applicable to unstable mobile users spread over a wide geographic area, including remote regions. The system is commercially feasible because it offers an almost continuous differential data update with high reliability at far range and low cost. The system accomplishes this objective by utilizing a geosynchronous commercial earth satellite relay for the communication link for the differential corrections. The differential correction data is condensed and formatted using spread spectrum techniques. The linearly polarized commercially relayed signal is received by the mobile user with a low cost, non-directional, non-stable circularly polarized antenna.
It is another feature of the present invention to provide a differential navigation system that operates with the navigation information from NAVSTAR GPS.
It is a further feature of the present invention to provide an improved differential navigation system wherein the differential data is transmitted from the reference receiver to the up-link earth formatting and transmitting unit using an earth satellite link.
It is another feature of the present invention to provide an improved differential navigation system wherein the navigation information upon which the differential data is based and the navigation information used by the mobile unit to compute location are the same information.